Silver halide film achieves its great speed in the case of the high speed variety by the process of development. The chemical reactions occurring in the development baths enable the achievement of ASA ratings which are of the order of hundreds. Known electrophotographic media are much slower in speed than silver halide film because the latent image, when formed electrostatically, cannot be improved by further processing. This latent image is not a chemical image but is a latent charge image, being electrostatic; hence its character is established by the electrical properties of the medium and the phenomenon which produced the image, i.e. light etc.
Although a true comparison of speed between silver halide film and an electrophotographic film cannot actually be made on the basis of A.S.A. or Din. ratings, due to the definition of A.S.A. rating, some general or roughly quantitative measure can be discussed. As stated, A.S.A. ratings of silver film can extend from tens or twenties to as high as several hundred and even as high as 1000 for special film. Even low A.S.A. film can be processed in such a manner as to change its initial A.S.A. rating upward by a multiple of ten or twenty. It can be said generally that the higher the speed of silver halide film, the coarser the grain. This comment should be kept in mind in view of description of the electrophotographic medium which will be given below, since, resolution of an image on silver halide film depends upon the size of the grains of silver produced in the emulsion during development.
The known methods of electrophotography depend upon (a) charging a photoconductor surface, (b) selectively discharging the surface by means of a light or other radiant energy pattern and (c) toning to develop the latent charge image. Instead of toning, in the case of a certain type of electrophotographic film which is disclosed in U.S. Pat. No. 4,025,339, the latent image may be read out by an electronic beam. The speed and sensitivity of a given electrophotographic medium is related to the ability of the photoconductor to accept and retain a charge and its ability to discharge selectively in response to light, for example. These characteristics are inherent in the constitution of the photoconductor and its method of manufacture.
Once the latent charge image has been established on the photoconductor the duration of the image will depend upon the ability of the photoconductive layer or coating to resist self-discharge. This ability is described by the dark decay characteristics of the photoconductor.
It has been known that a dielectric coating on a photoconductive medium will provide greater contrast than the same photoconductive medium without such coating and will enable longer retention of the latent image produced during exposure as taught, for example, by the Canon NP process ("Canography [Canon NP Process] in Electrophotography" by Matao J. Mitsui, IEEE Transactions in Electron Devices, Vol ED-19, No. 4, April 1972, pages 396-404). So far as known there has been no commercially successful medium which does not require the charging of the medium before its exposure. The inconvenience of the added step of charging and the expense of the accompanying requirement for apparatus to effect the charge when compared with a process that requires no charging are obviously great disadvantages.
It has been suggested to transfer the image from a photoconductive layer to an insulating layer but in those instances the voltage which has been available for generating the latent image has been only the charge applied before imaging and retained on the medium. In other words the medium must be charged before imaging.
There is one method of imaging which does not require prior charging but which has other advantages that have been eliminated by the invention in an unobvious manner. This method is detailed in the reference "Electrophotography" by Schaffert, The Focal Press, London, 1975, pages 172 and following.
In Schaffert reference is made to a technique in which electrostatic images are produced on the surface of a dielectric film member while the dielectric member is in contact with a xerographic member. U.S. Pat. No. 2,825,814 to Walkup is mentioned as the origin of the technology. The assembly of layers comprises a photoconductive layer on a conductive substrate such as metal or NESA glass which is transparent. The dielectric film member with a conductive backing such as a transparent metallic coating is placed in contact with the surface of the photoconductive layer. A high voltage of order of several thousand volts is applied between the conductive base of the photoconductive layer and the electrode. Simultaneously an optical image is projected onto the assembly, either through the back or front - whichever is transparent. According to the disclosure, after the brief exposure to light and the electric potential the light is turned off and the dielectric member separated from the photoconductor surface, the applied electric potential being maintained while this occurs.
The disclosure of the above identified publication recognizes that there is an advantage to this process because the dark decay characteristic of the photoconductor need not be as great as in the case where it must be charged initially, exposed and then be required to retain the charge.
So far as known, no technique of this type has been embodied in a commercial device. It is quite clear that there are several very important disadvantages to the technique and the structure which militate against the possibility of achieving practical results:
A. The use of high voltage. Keeping a voltage of several thousand volts applied to members which are to be used as described in dangerous and leads to the need for expensive equipment and insulation materials. PA1 B. The separation of the dielectric layer. Even at low voltages, stripping off a sheet member such as dielectric film is certain to produce breakdown of the gap and thereby deteriorate the latent image on the photoconductor and/or dielectric member. PA1 C. The presence of the air gap. The bringing together of the dielectric member and the photoconductor cannot help but produce an air gap, as indicated in the prior art disclosure. The charges from the photoconductor must therefore cross the air gap in order to settle onto the dielectric member. It is impossible for the air gap to be absolutely uniform as a result of which the transfer is uneven. There will be loss in the transfer because of the air gap in addition to unevenness.
According to this invention there is a constant voltage available which is independent of the surface potential of the photoconductive member, as required in other electrophotographic media, the constant voltage being applied between the ohmic layer and the surface electrode so that in effect the act of exposure enables the photoconductive layer to modulate the movement of carriers. The current source represented by the power supply furnishes large amounts of carriers for transport, millions of times more than would be available from usual techniques of charge and discharge by exposure to radiant energy. Furthermore, the voltage which need be used in the invention is substantially less than 100 volts, which is in contrast with the prior art technique described in the Schaffert publication that must use several thousands of volts.
In any system which attempts to utilize an electrode and a dielectric layer, the electrode and layer must be intimately connected and the dielectric layer must be intimately connected with the photoconductive surface. Any spacing or gap produces discontinuities and unevenness. Furthermore, stripping the dielectric layer off the photoconductive layer after imaging produces sparking or corona discharge and destroys the latent image or deteriorates the same. If the dielectric layer is bonded to the photoconductive surface, one must remove the electrode which means that the electrode must be removable, hence will give rise to a gap between electrode and dielectric during imaging. The disadvantages of this, as stated, are nonuniformity in charge distribution and likelihood of breakdown as well.
Applicants are aware of the work of H. Kiess of Zurich, Switzerland who conducted experiments in an attempt to increase the sensitivity of Electrophotographic media. Kiess used an assembly consisting of a photoconductive member which he describes as "prepared CdSe layers" and CdSe single crystals, but without further details, mounted on a grounding member and having an insulating film mounted thereon with a volatile conducting fluid serving as the electrode on top of the insulating film. Kiess charged the photoconductive layer and then brought his insulating film against the photoconductive surface to induce an image of the latent image from the photoconductor onto the insulating film. The volatile conducting fluid is connected to the grounding member to effect the transfer of charge to the insulating film without external application of voltage. When the volatile liquid evaporates, the film is stripped off the photoconductive layer. Then the insulating film can be toned to develop the image. Kiess succeeded, according to his report, to store images for several months before there was substantial loss in total surface charge. He claims to have achieved A.S.A. ratings of the order of 100 with some samples going as high as 300 A.S.A.
The invention obviates the need for pre-charging; eliminates the requirement to separate any layer from another to provide for development of the latent image; eliminates the problems of connecting the electrode in place and disconnecting it; eliminates all gaps either between the electrode and dielectric layer or between the dielectric layer and the photoconductive surface.
The speed and sensitivities of prior electrophotographic media, including the experimental ones described, are so low that the ability of the media to be exposed in nanoseconds if need be or to be discharged fully in similar times cannot be achieved and would not be expected. With respect to the ability of the electrophotographic medium to respond fully to radiant energy in nanoseconds, this is essential to a high speed film. Known photoconductive materials have extremely slow transit times, either because of the thickness of the required layers or because of the nature of the material. It requires highly intense light to achieve a large volume of carriers, even when assisted by external power sources; hence speeds of the general order of 500 to 1000 A.S.A. cannot be achieved. Further, one could not expect to be able to read electrostatic images from such materials with electron beams because the time for discharge of the surface charge is too great.
The transit time of the carriers in passing through the photoconductive layer of the electrophotographic medium of the invention is less than the carrier lifetime thereby sustaining the electric field during carrier travel. Further, the entire bulk of the photoconductive layer is depleted during use thereby ensuring uniform transit of carriers without any variations because of zones of opposite energy states. For example, if the carriers are electrons, as in the preferred example, there are no holes which form as a zone to make transit difficult.
Because of the ability of the electrophotographic layer to respond with extremely high speed and the sensitivity produced because of the external power supply producing a large volume of carriers, the medium can respond to the most minute amount of light to produce large numbers of carriers in a short time. Ideally the exposure time should be the time that it takes for the bulk of carriers to move through the thickness of the photoconductive member. In practice this time is of the order of at least microseconds. The speed of the electrophotographic medium under typical conditions has been calculated to be of the order of 30,000 A.S.A. using an approximation.
The electrophotographic medium of the invention preferably uses as its carrier modulating portion a cadmium sulfide film constructed as disclosed in said U.S. Pat. No. 4,025,339, but other electrophotographic films having the general attributes could be utilized to achieve the benefits of the invention.